Tag Archives: hydra

How immortal Hydras regenerate severed heads

A tiny immortal hydra. Credit: Creative Commons.

Hydras, tiny aquatic animals, are some of the most intriguing creatures in the animal kingdom. For one, they’re virtually immortal. Because their stem cells have the capacity for unlimited self-renewal, unless the hydra is eaten or physically destroyed by an external force, it could theoretically live forever. It does know death from old age.

Even when its head is severed, the hydra will grow one back — its will to live is truly unmatched. Now, researchers in California have found their secret, and it’s all in their genes. Or rather, in how they’re regulated, known as ‘epigenetics’.

Hydras, which are related to corals, sea wasps, and jellyfish, are asexual, meaning they can reproduce on their own without the need for a mate. Instead, they reproduce by growing buds on the surface of their bodies, miniature clones of the original individuals that simply break away once they are mature. Research has found that animals that reproduce later on and less frequently tend to live longer. Hydras, however, begin to reproduce almost immediately after they form.

In 2012, researchers at the Zoological Institute of Kiel University found that the key to the hydra’s longevity is the FoxO gene, which was previously linked with aging in humans. The researchers found that FoxO plays a fundamental role in the maintenance of stem cells

The hydra is basically a bag of stem cells. In fact, it is an adult that continues to churn out embryonic cells, which sort of makes it a perennial embryo. Because the genes that regulate development are constantly switched on, they are constantly rejuvenating the body.

Researchers at the University of California Irvine thought the same mechanisms found in reproduction may also explain the death-defying act of head regeneration. But, as they reported in the journal Genome Biology and Evolution, the two processes are distinct.

“One exciting finding of this work is that the head regeneration and budding programs in Hydra are quite different,” said the paper’s lead author, Aide Macias-Muñoz. 

“Even though the result is the same (a hydra head), gene expression is much more variable during regeneration. Accompanying dynamic gene expression is dynamic chromatin remodeling at sites where developmental transcription factors bind,” the scientist added.

Hydra forming a new head after it was cut off. Credit:  Ulrich Technau/PNAS.

The biologists mapped more than 27,000 elements that are active in one or more segments of the hydra’s body or regenerating tissue. They used a method called ChIP-seq that analyzes how proteins interact with DNA, so they could see which sections of the genomes are turned on or off during a particular stage of development, such as head regeneration. They found 2,870 regions of the hydro genome that drive head regeneration but are not all necessarily active during budding, including “enhancer” genes, genomic sequences that play a key role in regulating tissue-specific gene expression levels.

According to Science Alert, the researchers identified a family of genes known as Fos that was very active in head regeneration. The same genes were previously associated with regenerative processes in other species, such as fish, salamanders, and mice.

Many of the hydra’s genes involved in regenerating its severed head are also present in humans. It’s just that ours are switched off, while theirs are switched on by environmental cues — that’s the essence and power of epigenetics. If humans had access to the same genetic programming, could this be a fountain of youth? That’s one rabbit hole the researchers haven’t dared enter. But by detangling these mechanisms, researchers may further elucidate other key aspects of animal evolution and development, such as that of the nervous system, the authors noted in their study. 

Researchers map the genetic mechanisms that makes hydras ‘immortal’

Researchers are especially interested in hydra’s ability to regenerate its nervous system, which could new therapeutics for treating trauma or degenerative disease in humans.

Image credits Stefan Siebert / UC Davis.

The tiny freshwater invertebrate known as the hydra, while definitely less scary than its mythological counterpart, regenerates damaged cells and tissues. This ability is so poignant that, were you to cut a hydra in half, it would regrow its body and nervous system in a matter of days.

Trying to understand exactly how it does this, researchers at the University of California have traced the evolution of the hydra’s cells throughout its life, finding three lines of stem cells that differentiate into nerves, muscles, or other tissues.

Life renewed

“The beauty of single-cell sequencing and why this is such a big deal for developmental biologists is that we can actually capture the genes that are expressed as cells differentiate from stem cells into their different cell types,” says Celina Juliano, assistant professor in the UC Davis Department of Molecular and Cellular Biology and lead author of the study.

Juliano’s team sequenced RNA transcripts of 25,000 single hydra cells to follow the genetic trajectory of nearly all of the animal’s differentiated cell types. The study thus creates a high-resolution map of the entire developmental path of the hydra’s cells.

Hydras continuously renew their cells from stem cell populations, the team explains. Based on the analysis of sets of messenger RNA molecules (transcriptomes) retrieved from individual cells and groups of cells (based on shared expressed genes), the team separated these stem cells into three different lineages. They could then build a decision tree showing how each lineage matures into different cell types and tissues. For example, the interstitial stem cell lineage produces nerve cells, gland cells, and the stinging cells in the animal’s tentacles.

“By building a decision tree for the interstitial lineage, we unexpectedly found evidence that the neuron and gland cell differentiation pathway share a common cell state,” said Juliano. “Thus, interstitial stem cells appear to pass through a cell state that has both gland and neuron potential before making a final decision.”

The molecular map also allowed Juliano and colleagues to identify the genes that influence these decision-making processes, which will be the focus of future studies.

The team hopes that their work will allow developmental biologists to understand regulatory gene networks that control the early evolution of the hydra, networks that they say are shared among many animals, including humans. Understanding how the hydra regenerates its entire nervous system could thus help us better understand neurodegenerative diseases in humans.

“All organisms share the same injury response pathway but in some organisms like hydra, it leads to regeneration,” said coauthor and graduate student Abby Primack. “In other organisms, like humans, once our brain is injured, we have difficulty recovering because the brain lacks the kind of regenerative abilities we see in hydra.”

The paper “Stem cell differentiation trajectories in Hydra resolved at single-cell resolution” has been published in the journal Science.

Hydras rip their skin to open their mouths

Hydras are tiny freshwater animals which trap their prey with a set of tentacles. After they do so, they open their mouths to eat the prey. Now, that wouldn’t be a big challenge for most animals, but hydras have to actually tear up their skin to open their mouth.

Hydra vulgaris is shown with its two tissue layers transgenically labeled: ectoderm (outer layer) in green; endoderm (inner layer) in magenta.
Credit: Callen Hyland

Hydras are quite strange creatures. We’ve written before about how they seem to be virtually immortal and how studying their genes could pave the way for our own longevity. Hydras also have the ability to regenerate their tissue very quickly. But it’s not all fun and games for hydras – something as simple as opening the mouth can be a tearing task.

The hydra’s mouth is sealed into a continuous sheet of tissue and opening it requires dramatic morphological changes. Researchers chemically tagged some of these cells and learned that these cells would rather break apart than move.

This sequence of photos shows how Hydra can rapidly open its mouth to its fullest extent after only 13 seconds.
Credit: Image courtesy of University of California – San Diego

“It’s fascinating that Hydra has to tear a hole every time it opens its mouth,” said Eva-Maria Collins, a biophysicist at the University of California, San Diego and lead author of the study. “And that this process happens so quick; this was the first indication to us that mouth opening did not involve cellular rearrangements.”

At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. The hydra only opens its mouth when it’s sure it grabbed some prey, and when it wants to spit any remains. When they added magnesium chloride (a muscle relaxant) in the water, hydras didn’t open their mouth at all.

“The fact that the cells are able to stretch to accommodate the mouth opening, which is sometimes wider than the body, was really astounding,” says Collins. “When you watch the shapes of the cells, it looks like even the cell nuclei are deformed.”

Because the hydra is such a simple animal, it offers researchers the possibility to study the process in detail.

“We can try to understand what look like very complicated processes in the living animal with relatively simple physics,” Collins says.

However, why an animal would develop such a complicated and tough way to open its mouth still remains anyone’s guess. We also don’t know what the physiological effects are – does the hydra consume more energy to “fix” its mouth each time it gets broken? Does it save energy by having it sealed shut most of the time? Collins and her team will try to answer these questions in the future.

Journal Reference:

  1. Jason A. Carter, Callen Hyland, Robert E. Steele, Eva-Maria S. Collins.Dynamics of Mouth Opening in Hydra. Biophysical Journal, 2016; 110 (5): 1191 DOI: 10.1016/j.bpj.2016.01.008

A life in a pond – amazing timelapse video

If you’ve ever looked at a pond and thought “Well, not much really going on here”, you couldn’t have been further from the truth. As this fantastic video created by Daniel Stupin shows, a lot is happening even in apparently still ponds.

The hidden life in pond water from Daniel Stoupin on Vimeo.

If you’re interested in reading more about he creates this kind of video (and microphotography), it’s well worth checking out his blog – lots of great info there.

Ok, so what are we seeing here exactly?


Water fleas are not related to fleas at all – they are small crustaceans virtually ubiquitous in inland aquatic habitats. They have a single eye, reproduce mostly asexually, and can survive extremely harsh conditions and habitats.

Bryozoa are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they are filter feeders that sieve food particles out of the water. Almost all live in colonies made up of animals with specialist roles that could not survive on their own. Freshwater species, like the ones in the video, are hermaphrodites.

Mayflies are famous (or infamous?) for their extremely short lives, which are oftentimes present in poetry. They are part of an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies.

Mosquitoes are well known by almost everybody, but not so many people know that out of the 3500 mosquito species, only a few species actually suck on blood. Most of them feed on pollen, and even in the species which do suck blood, it’s only the pregnant females who need the protein to hatch their eggs.

Water mites are a group of mites containing over 5,000 species found in freshwater habitat. They are related to spiders.

Ostracods are a remarkable old group, which have recently provided the oldest example of parental care – 450 million years ago.

Ciliates, which you can also see in this video, are not technically animals, but rather single-celled protazoans, although the larger ones can be longer than some of the smaller animal species Stoupin has captured.

Hydra are radially symmetric predators which look like plants more than animals. A while ago, we told you about a hydra species which apparently doesn’t age, and is the world’s only immortal animal.


Longevity gene that makes the Hydra immortal identified

HydraThe Hydra is a tiny animal that can be found in just about any freshwater pond, just a few millimeters long, that has attracted the attention of scientists for years now due to its extraordinary regenerative abilities. The Hydra is consider to be biological immortal – it does not die from old age – although a scientific consensus has yet to be reached. Scientists studying the polyp Hydra claim they now know how the creature escapes senescence after they found a key gene. This gene is also believed to be linked with aging in humans.

The animal’s potential immortality is made possible by its reproductive system. The Hydra is an asexual being and doesn’t mate, instead it reproduces by producing buds in the body wall, which grow to be miniature adults and simply break away when they are mature. Popular scientific consensus has found that animals that reproduce later on and less frequently tend to live longer. The Hydra, however, begins to reproduce almost immediately.

The forever young Hydra

Biology Professor Daniel Martínez at first was extremely skeptical of the claim that Hydras were biological immortal. He set out to disprove this assumptions and cultured tens of specimens, which he kept in isolation waiting for them to die. It’s already been four years and no specimen has yet succumbed from natural causes. For an animal of this size, nature dictates that it should have died long before.

Returning to the Hydra’s reproductive system. For this vegetative-only reproduction to work,  each polyp contains stem cells capable of continuous proliferation. “Hydra is a bag of stem cells,” Martinez says. “It is an adult that is produced by embryonic cells, so it is really a perennial embryo. The genes that regulate development are constantly on, so they are constantly rejuvenating the body.”

The gene that makes it all happen

As humans age, as well as many other complex biological lifeforms, stem cells lose the ability to proliferate and thus to form new cells. This causes tissue decline, which is why muscles get weakened with old age for instance. Influencing the processes that go with aging has been a goal for scientists science the advent of modern science. The Hydra might potentially have the ability to open new doors, especially after the latest research from scientists at the University Medical Center Schleswig-Holstein (UKSH) who recently found the gene that causes Hydra to be immortal – the FoxO gene.

Now, the gene itself isn’t something new. It’s been known by scientists for years and is present in all animals, and humans as well. However, until now it was not known why human stem cells become fewer and inactive with increasing age, which biochemical mechanisms are involved and if FoxO played a role in aging.

The German researchers genetically modified a batch of polyps such that they obtained Hydras with: no FoxO gene, deactivated FoxO gene and enhanced FoxO gene. Their  findings show that the animals with no FoxO gene have significantly fewer stem cells. Interestingly, the immune system in animals with inactive FoxO also changes drastically.

“Drastic changes of the immune system similar to those observed in Hydra are also known from elderly humans,” explains Philip Rosenstiel of the Institute of Clinical Molecular Biology at UKSH, whose research group contributed to the study.

The researchers go on to note that there’s a link between FoxO and aging in humans.

“Our research group demonstrated for the first time that there is a direct link between the FoxO gene and aging,” says Thomas Bosch from the Zoological Institute of Kiel University, who led the Hydra study. Bosch continues: “FoxO has been found to be particularly active in centenarians — people older than one hundred years — which is why we believe that FoxO plays a key role in aging — not only in Hydra but also in humans.”

Fighting aging in humans

That’s to say that FoxO has been proven to be linked with aging in humans, since testing such a hypothesis would require genetic modification of actual people. Remember, that the Hydra is an extremely primitive organism – immortal as it may be. Imagine that if you  take a hundred hydra, make a cell suspension, dissociate all the tissue, put it in a centrifuge, make it into a bowl, you’ll soon see how from those cells, somehow they bind together and you’ll get a couple of new hydras!

It’s been tested with mice, however, and apparently though they didn’t make them immortal, the enhanced gene therapy did in fact prolonged their lives considerably.

Here’s the take away: FoxO gene plays a decisive role in the maintenance of stem cells, according to these findings. I may be overstating this, so someone please correct me if so, but it also means that the FoxO gene determines life span in all animals, from simple being to the top of the food chain humans. So what’s the key to longevity? The maintenance of stem cells and the maintenance of a functioning immune system. If you you’ve got these two in cue, you’ve got nothing to worry about – except freak accidents!

I recommend you also read one of my earlier pieces which also discusses another immortal animal – that’s right, you’re own backyard flatworm. This little puppy can regenerate its cells indefinitely thanks to the telomerase enzyme, which keeps DNA telomeres from shrinking and thus also keeps cell regeneration indefinite.

The findings of the German scientists were documented in the journal Proceedings of the National Academy of Sciences(PNAS).

Meet the world’s only immortal animal

If you’re thinking McLeod, you couldn’t be further from the truth. What you have to do is think small; not microscopic, just big enough to see with your naked eye. Turritopsis nutricula is a hydrozoan, and it’s considered by scientists to be the only animal that cheated death.

hydrozoaSolitary organisms are (according to current belief) doomed to die, after they completed their life cycle. But Hydrozoa don’t live by normal rules. Nope, Hydrozoa are a huge class of predatory animals that live mostly in saltwater, closely related to jellyfish and corals. Eggs and sperm from an adult jellyfish (medusa) and they then develop into polyp stage. Medusae evolve asexually from polyps.

Still, our Turritopsis nutricula (can I call it Joe?) managed to find a way to beat that. What these little folks do is they revert completely to a sexually immature, colonial stage after they reach sexual maturity. They’re even cooler than that. When they’re young they’ve got only several tentacles, but at a mature stage, they get to 80-90 of them.

They’re able to return to polyp stage due to a cell change in the external screen (Exumbrella), which allows them to bypass death. As far as scientists have been able to find out, this change renders the hydrozoa virtually immortal.

Edit: This species is now classified as Turritopsis dohrnii – but since I wrote this article, biologist changed their mind and gave it a new name and a new classification. Everything else remains the same though.